To assess the congruence between two earlier independent phylogenetic studies on Bacillus thuringiensis strains and on Bacillus-related species and the single, all-encompassing, phylogenetic tree presented here inferred from the combination of the two earlier datasets. A dendrogram was constructed using a combination of the data from our previous studies on 16S rRNA gene fingerprinting of 86 B. thuringiensis strains and of 77 species of Bacillus and related taxa. It revealed that all B. thuringiensis strains were clustered together in four distinct groups at a DNA similarity rate of 93%, except two serovars, bolivia and finitimus. Four bacilli species, Paenibacillus alvei, P. azotofixans, B. lentus and B. licheniformis, share a DNA similarity rate of 92% with Bt Group IV. Most, but not all, B. thuringiensis strains could be grouped together based on the DNA similarity rate. They were also very close to some other bacilli species. The combined phylogenetic study presented here, inferred from 16S rRNA gene restriction fragment length polymorphism, is congruent with two earlier independent phylogenetic studies, one on B. thuringiensis and the other on bacilli species.

The genetic relationships of Bacillus cereus and of the Bacillus thuringiensis serovars were assessed from measurements of DNA reassociation. A study of 8 to 10 strains each of 13 of the most commonly encountered serovars revealed that the levels of intragroup DNA relatedness for most serovars ranged from 90 to 100%. In contrast, B. thuringiensis serovars canadensis and kenyae consisted of two DNA relatedness groups, each of which exhibited levels of intragroup relatedness of 80% or higher and levels of intergroup relatedness of 60 to 70%. Analyses of DNA relatedness performed with all of the serovars revealed that the taxa were segregated into 11 phena differentiated from each other at about the 65% level; within each phenon the level of relatedness was 80% or higher. Three phena contained strains belonging to more than one serovar; B. thuringiensis serovars alesti and dendrolimus clustered in phenon 1, serovars aizawai, kurstaki, galleriae, and morrisoni clustered in phenon 7, and serovar darmstadiensis and some strains of serovar kenyae clustered in phenon 11. The levels of DNA relatedness between B. cereus and B. thuringiensis strains ranged between 65 and 70%. My results suggest that many of the B. thuringiensis serovars are genetically distinct but closely related.